Participation in daily physical activity programs by young children is currently recommended as a means of promoting bone health. Results from adult studies indicate that beneficial effects of either physical activity or calcium (Ca) intake may be apparent only when both factors are present. Our results in infants indicate that physical activity combined with a low Ca diet may he detrimental in terms of bone mass accretion. The overall objective of this study is to determine whether Ca intake modifies the bone response to activity in young children 3 to 4 years of age. Our hypotheses are that 1) the increase in bone mass resulting from a physical activity program will be more pronounced in children randomized to Ca supplement compared to the increase in children randomized to receive a placebo; and 2) that 12 months following cessation of the activity program, bone mass will remain higher in children randomized to gross motor activity compared to children randomized to fine motor activity, and that the beneficial effect of Ca supplementation will persist only among children randomized to gross motor activity. These hypotheses will he tested in a randomized 2 x 2 factorial trial in 3 to 4 year old children. Children will he randomized into either a gross motor or fine motor activity program that will be conducted in child care centers 5 days a week for 1 year. Each child will be further randomized into either a Ca supplement (1 g/d) or placebo group. The primary outcomes are bone mass accretion and changes bone mineral density, which will be determined by dual energy x-ray absorptiometry at the beginning and end of the study. Activity assessments throughout the study period will enable us to determine whether participation in the gross motor activity group also increases spontaneous activity in these children. Anthropometric measurements and dietary information will allow us to statistically control for these potential onfounders. Additional bone mass and physical activity measurements will be obtained 12 months following completion of the program to determine if there are long term effects of these interventions on bone mineral density and activity. A finding of beneficial effects of either Ca supplementation or physical activity either independent of each other, or in combination, will provide the groundwork to devise prevention strategies within the educational system for optimizing bone health beginning early in life. However, we may find that increased physical activity, in the presence of a low to moderate Ca intake, may have a detrimental effect on one mass accretion during periods of rapid growth.